1. Field of Invention
The present invention relates to a method and system for power control by interference prediction with error margin for wireless Internet protocol (IP) networks.
2. Description of Related Art
The future generations of wireless networks must accommodate a growing demand for data packet services. High-speed packet services are necessary for wireless data packet communications, such as Internet protocol (IP), which can provide efficient access to remote networks and servers for telecommuters and to facilitate wireless multimedia services such as voice, audio, still-image and video.
Currently, cellular systems employ frequency reuse techniques whereby multiple cells in a network, known as co-channel cells, use the same set of frequencies. The frequency reuse factor of a cellular system is given by the variable N, where N is the cluster size which describes the number of cells which collectively use the complete set of available frequencies. The cluster size N should be chosen according to the amount of interference a mobile or base station can tolerate while maintaining a sufficient quality of communications. The total capacity of a cellular system is inversely proportional to the frequency reuse factor. Thus, decreasing the frequency reuse factor (i.e., the cluster size N) is essential for improving the capacity of cellular networks.
A major limiting factor in the performance of cellular wireless systems is interference. In particular, while frequency reuse improves capacity, it produces co-channel interference. Co-channel interference cannot be corrected by increasing the transmission power of the transmitter because increasing the transmission power raises the interference in neighboring co-channel cells. To reduce the co-channel interference, the co-channel cells must be physically separated by a minimum distance to provide sufficient isolation.
In order to improve the performance of time-division-multiple-access (TDMA) wireless networks and ensure that each base station and mobile terminal transmits the smallest power necessary to maintain a good quality radio link, power control within a network has become essential. Power control not only helps prolong battery life for the mobile units, but also can dramatically enhance the signal-to-interference-plus-noise ratio (SINR) in the system, and thus its error performance and capacity. Accordingly, dynamic transmission power control has been widely studied and practiced to combat and manage interference in cellular radio networks.
Known power control techniques for wireless networks can be categorized as either signal-based and signal-to-interference-ratio (SIR) based. In signal-based power control algorithms, the transmission power is adjusted based upon a received signal strength, which in turn depends upon the path loss, shadowing and fading of the radio link between the transmitter and the receiver. In contrast, SIR-based power control adjusts the power according to the ratio of the power level of the signal to the power level of the co-channel interference (possibly including noise). Studies have shown that SIR-based power control out performs signal-based power control, although the former involves a more complex implementation.
A drawback to both of these power control techniques is that they are applicable mainly to circuits switched connections having a relatively long holding time. Accordingly, these methods utilize iterative algorithms that require the re-adjustment of transmission power over the entire duration of a circuit switched connection and implicitly or explicitly, assume a relatively long call duration. However, the nature of the data traffic and packet-switched networks is bursty, which is fundamentally different from that of circuit-switched networks. For example, in TDMA packet-switched networks, time is divided into slots where the slot size is appropriately chosen to support the applications while controlling the protocol overhead to achieve efficient bandwidth usage. Typically, each data message is divided into a number of packets, each of which can be transmitted in one time slot. As in typical IP wireless networks, the message length (in terms of number of the number of packets) varies randomly from message to message.
Due to the bursty nature of the traffic and irregular transmission schedule inherent in packet-switched data networks, the traditional power control techniques, such as the above-described existing signal-based and SIR-based techniques, do not perform well. The above-described iterative power control methods for circuit switched networks are inefficient for packet-switched wireless network. Accordingly, new techniques have been developed to accommodate such bursting packet-switched network traffic. As described in co-pending U.S. patent application Ser. No. 09/273,125 filed on Mar. 19, 1999, one such system uses a Kalman filter to predict interference power and adjust a transmission power to achieve a target SINR performance.
Such a method relies on the fact that typical wireless networks allow multiple contiguous time slots to be used by a base station or mobile terminal for transmitting a message, and therefore a temporal correlation exists of the interference power between successive time slots. The temporal correlation allows the use of predictive methods, such as Kalman filters, to estimate the interference power in subsequent time slots. Accordingly, a signal path gain parameter and a prediction of the interference power for a future time slot can be used to calculate the power level for the future time slots in order to met a target SINR. However, the interference power prediction can become very inaccurate when each message consists of very few packets, such as where one packet is transmitted in one time slot. In addition, the performance gained by the method reduces when delay is incurred in interference measurements and in the forwarding of power control information from receivers to transmitters. Both of the situations of short messages and control delay are expected in certain wireless IP networks.
Accordingly, there is a significant need for a more efficient power control method for wireless networks.
The present invention provides a method and system for providing power control in a wireless packet switched network using interference prediction including an error margin. In particular, the method can measure an interference power and a path gain between an intended receiver and transmitter. Based upon the past performance of the network, a future interference value may be predicted by using a prediction algorithm, such as a Kalman filter. Furthermore, based upon the prior accuracy of the interference prediction algorithm, the method can also estimate an error margin for the predicted interference values. Finally, a transmission power for the transmitter can be calculated using the predicted interference power, the estimated error margin for the predicted interference power, the path gain, and the target SINR. Since the effects of short message and control delay have been reflected by the error margin, the enhanced power control method with such an error margin provides accurate interference power prediction, and thus yields a performance gain.